1. Field of the Invention
The invention relates to a method for the manufacture of fiber-reinforced plastic compositions. It furthermore relates to an apparatus for the practice of the method.
2. Related Art
A method and an apparatus for the manufacture of fiber-reinforced plastic compositions using a plasticizing extruder is disclosed in U.S. Pat. No. 6,238,733 (claiming priority to DE 198 36 787 A1), which is incorporated herein by reference in its entirety. The method and the plasticizing extruder described therein have proven practical. However, there is still room for improvement for special fiber strands and fiber materials, especially in the manufacture of fiber-reinforced plastic compositions (plasticates) from fiber fleeces or fiber mats and a fluid plastic film. In order to achieve the same effect as with the method of DE 198 36 787 A1, various methods have been used in the past. For example, natural fibers have been fused from semi-finished products (pellets), e.g., as rodlet granules, or these fibers have been fed through a fiber stuffing unit into the impregnating apparatus. This results in a very poor quality of impregnation. Fiber fleeces can also be shot through with thermoplastic fibers and pressed in the shaping process. The disadvantages in the known methods and in the use of semi-finished products are the high cost and the additional waste in the transformation process.
In regards to the fusing of fiber rodlets, mention must be made of low fusion performance in the case of gentle fusion, and the great harm done to the fibers and the poor fiber impregnation obtained when operating with fiber plugging units.
The invention is addressed to the problem of offering a method whereby the disadvantages discussed above can be avoided, and especially to increasing and better defining the fiber content in the plasticate, as well as improving performance in the production of plasticates from fiber fleeces and fiber mats. A further object of the invention is the creation of an apparatus with a plasticizing extruder for the practice of the method, with an adapted geometry of the extruder casing and of the corresponding screw elements.
With the method and the apparatus with a plasticizing extruder according to the present invention, a number of advantages and improvements are achieved. For example, a higher and more precisely defined fiber content can be achieved in the end product as well as good impregnation of all fibers. Another example is that a higher throughput is achieved, which allows for a very great increase in output in the creation of the plasticates. Another example is that without producing semi-finished products, the impregnation and production of the plasticate can be performed by a direct method That is, directly from raw bales with a fleece producing machine, where the fiber fleeces do not need to satisfy stringent requirements. Yet another example is that with the plasticate technology according to the invention, continuous pressing is possible, allowing ribs and nubs to be formed in the product.
It should also be mentioned that yet another advantage of the invention is that the weight per unit area or weight per meter of the fiber fleece or mats and/or of the combination is continuously determined before entry into the extruder. Also, the weight of the fiber in the plasticate and the plasticate weight are kept within close tolerances. Additional advantages are that the fiber fleece and/or fiber mats in band form, presented in coils or rolls, can be fed to the extruder, and by a manual or automatic change of coils or rolls they can be joined together to form an endless fiber fleece or fiber mat for delivery into the extruder, or the fiber fleece can be formed directly on a conveyor belt by spreading the fibers onto it and can be fed to the extruder. Also, the fiber fleece and/or the fiber mat can be dried directly within the line before the line is drawn into the extruder. Finally, there is also the possibility of drying the fiber fleece before it is drawn into the extruder, and the fiber fleece or fiber mat web can serve additionally as a conveyor belt for sprinkling with staple fibers, long-fiber granules, granules, recyclate chips or fillers.
The solution of this problem as to the method is to be seen in connection with three embodiments, as follows: In the first embodiment the problem is solved by the fact that endless fiber fleeces or fiber mats made of natural, chemical, vitreous, mineral, carbon, chopped or metal fibers and possible combinations thereof, in the form of a continuous web of the approximate width b equal to the length of the feed slot, are fed with the fluid plastic through a slot-like inlet in the plasticizing extruder, and that the fiber fleeces or fiber mats are introduced with a wrap-around in the enlarged area of the screw cylinder into the grooves and onto the lands of the feed shaft.
In the second embodiment, by the fact that endless fiber fleeces or fiber mats made of natural, chemical, vitreous, mineral, carbon, chopped or metal fibers and possible combinations thereof, in the form of a continuous web of the approximate width b equal to the length of the feed slot, are fed with the fluid plastic through a slot-like inlet in the plasticizing extruder, and that the fiber fleeces or fiber mats are introduced with a wrap-around in the enlarged area of the screw cylinder into the grooves and onto the lands of the feed shaft are forced by strippers into the grooves and onto the lands of the feed shaft, and in the third embodiment, in that endless fiber fleeces or fiber mats made of natural, chemical, vitreous, mineral, carbon, chopped or metal fibers and possible combinations thereof, in the form of a continuous web of the approximate width b equal to the length of the feed slot, are fed with the fluid plastic through a feed slot into the plasticizing extruder, and that the fiber fleeces or fiber mats are introduced after a wrap-around in the enlarged area of the screw cylinder into the grooves and onto the lands of the feed shaft and tandem shaft.
For the drawing-in of fiber fleeces and/or fiber mats it is generally advantageous that the casing likewise increases in diameter in the area of the second screw shaft. The diameter enlargement in the feed area continues downstream to a greater or lesser extent into the closed area, depending on the fiber structure and impregnation ratio. Particularly if the fleeces and/or mats in band form are of great tensile strength, the impregnating process is improved by operating without strippers, because the molten plastic band/plastic films are fed in in the feed area and compressed by the fiber band. If the fleece and/or mats are of low tensile strength, however, it is advantageous to operate with a stripper on the feed shaft and to provide for a diameter enlargement in the feed casing for both screw shafts, while the diameter increase in the area of the second shaft can be less and can be disposed eccentrically, especially in the case of the feed shaft. That means that to prevent material build-up onto the lands of the feed shaft due to the increase of the diameter of the casing while the feed shaft is rotating, the screw housing bore is reduced to the screw diameter before any further pickup of the molten plastic band. This is achieved preferably with an inserted stripper bar. The stripper bar can also be made with a reduced gap between it and the screw lands.
For the performance of these operations the apparatus consists of a plastic melting extruder and a plasticizing extruder, and it is furthermore characterized according to one embodiment by the fact that, for the introduction of endless fiber fleeces or fiber mats of natural, chemical, glass, mineral, carbon, chopped or metal fibers into the plasticizing extruder, the infeed opening is parallel to the extruder axes and is in slot form over the feed shaft, the feed slot length being made with approximately the width b of the fiber fleece or fiber mats, the diameters D, increased by 2 to 20 mm and preferably disposed eccentrically, of the housing bores are again provided, after the wrap-arounds, with the otherwise common diameter d, or strippers are disposed on the tandem shaft and/or on the feed shaft following the wrap-arounds.
In another embodiment of the present invention, the method is further characterized in that the weight per unit area or the weight per meter of the fiber fleece or both weights is continuously weighed before entry in the plasticizing extruder and the percentage fiber content by weight in the fiber-reinforced plastic composition and the fiber-reinforced plastic composition weight are held within close tolerances by a rearwardly directed cascade control strategy, a forwardly directed cascade control strategy, or a rearwardly and forwardly directed cascade control strategy.
In another embodiment of the present invention, the method is further characterized in that the endless fiber fleeces are made from raw fiber bales in a fleece production apparatus and are fed therefrom directly to the plasticizing extruder.
In another embodiment of the present invention, the method is further characterized in that the ends of the fiber fleece or fiber mat are attached to an endless fiber fleece or fiber mat for feeding into the extruder with a manual or automatic spool/roll change.
In another embodiment of the present invention, the method is further characterized in that the fiber fleece is formed by spreading the fibers directly on a conveyor belt and is fed to the extruder.
In another embodiment of the present invention, the method is further characterized in that at least one of the fiber fleece and the fiber mat is dried directly within a line before being drawn into the extruder.
In another embodiment of the present invention, the method is further characterized in that at least one of the fiber fleece and fiber mat is preheated within a production line before it is drawn into the extruder.
In another embodiment of the present invention, the method is further characterized in that the fiber fleece web or the fiber mat web additionally serves as a transport web for staple fibers, long fiber granules, granules, recyclate chips or fillers.
In another embodiment of the present invention, the apparatus is further characterized in that the endless fiber fleece or the endless fiber mat is guided in the infeed opening over a rounded entry wall of the infeed slot.
In another embodiment of the present invention, the apparatus is further characterized as having a diameter reduction downstream from the infeed opening that terminates in a spiral in the direction of rotation.
In another embodiment of the present invention, the apparatus is further characterized in that a feed-slot jaw with an oscillating drive is disposed for movement on the entry end of the infeed opening.
In another embodiment of the present invention, the apparatus is further characterized in that the feed-slot jaw is thermally insulated from the housing.
In another embodiment of the present invention, the apparatus is further characterized in that the temperature of the feed-slot jaw can be reduced below the tackiness temperature of the fiber fleece.
In another embodiment of the present invention, the apparatus is further characterized in that strippers are provided in the feed-slot.
In another embodiment of the present invention, the apparatus is further characterized in that the strippers are exchangeable.
In another embodiment of the present invention, the apparatus is further characterized in that the strippers are at an interval away from screw lands which is smaller than an interval in the enlarged housing bores.